Photoinduced oxidation of water in the pyridine-water complex: Comparison of the singlet and triplet photochemistries

It has recently been shown that low-lying dark charge-separated singlet excited states of nπ∗ and ππ∗ character exist in the hydrogen-bonded pyridine-water complex in addition to the familiar nπ∗ and ππ∗ excited states of the pyridine chromophore. The former have been shown to promote the transfer of a proton from water to pyridine, resulting in the pyridinyl-hydroxyl radical pair. In the present work, the potential-energy surfaces of the triplet excited states of the pyridine-water complex have been explored with the same ab initio electronic-structure methods (ADC(2), CASPT2). Minimum-energy reaction paths for excited-state H atom transfer, energy surfaces in the vicinity of the barrier for H atom transfer, as well as multistate surface crossings have been characterized. The photochemical reaction mechanisms on the singlet and triplet potential-energy surfaces are compared, and their relevance for photoinduced water oxidation with the pyridine chromophore are discussed.